AJAX, a shorthand for Asynchronous
JavaScriptAnd XML is a web development technique that has the potential to blur the line between web-based and desktop applications. AJAX technique strives to achieve this by marrying the agility of a graphical user interface of desktop applications with the benefits of the Web. The proven success of AJAX based projects like
Google Map, Orkut, Google Suggest stand testimony to the fact that AJAX is more than just a catchy abbreviation. In short AJAX is an emerging concept space that is crying out for innovation.

The history of AJAX can be traced back to the introduction of an ActiveX component by Microsoft, in Internet Explorer 5 (1999).
This object was used to create Outlook Web Access. Soon Mozilla 1.0 and Apple Safari 1.2 followed the suit by the introduction of an object called XMLHttpRequest. This little-used object and the basic concepts were covered in a W3C standard; The DOM Level 3 Load and Save Specification (April 2004).
However the term AJAX was coined by Jesse James Garrett in a paper entitled AJAX: A New Approach to Web Applications, published in
www.adaptivepath.com, on 18 Feb 2005.
Google was apparently among the first to realize and start exploiting the power of AJAX in its various projects like Gmail , Google Map etc. This spurred the term ‘AJAX’ to become a buzzword in the industry

AJAX stands for Asynchronous JavaScript And
XML. In a nutshell, it is the use of a JavaScript object to communicate
to the server asynchronously.

Traditionally, web applications transfer a complete form or a webpage to the server, which in turn validates the data and renders a response that consists of a full HTML markup, back to the browser. The requirement for a full page post back, whenever the user interface has to be changed consumes lot of bandwidth apart from significantly hindering the performance and increasing the waiting time, to the extent that critics dubbed the term WWW - as World Wide Wait. AJAX can change all this.

An AJAX application eliminates the start-stop-start-stop nature of interaction on the Web by introducing an intermediary layer
i.e. an AJAX engine, between the user and the server. AJAX makes updated information available to the browser without refreshing the browser by asynchronously exchanging and processing small amounts of data with the server behind the scenes and by allowing traditional web pages to make in-page server requests.

To be more specific - In an AJAX enabled application , the page is loaded only once,
i.e. the first time it is requested, along with the HTML,CSS,
JavaScript – i.e.. the AJAX engine, that constitute the page. All requests to the sever are then sent as JavaScript calls to this AJAX engine which in turn requests information from the server asynchronously.
XML is used as the service payload to carry data back and forth. Upon completion of a request JavaScript may update or modify the Document Object Model (DOM)
of the HTML page based on the resulting XML document. Only the affected portions of the HTML are re-rendered on the HTML page. This leads to a much much more responsive interface that produces a feeling that the information displayed is instantaneous.
AJAX, technology can enrich user experience for shopping, working, planning, corresponding and navigating online, among a bouquet of other benefits.

Even before the term AJAX and the component XMLHTTPRequest came into being, web
application developers were already displaying reloading or updating a page dynamically, without full page reloads with the help of IFRAME or other similar technologies.
So AJAX is not a new technology all together, rather, it is more of a pattern that
allows existing technologies like, XML, JavaScript, HTML, CSS to come together and get used in a new way.